Braking systems for vehicles (eg. trucks, trailers and the like) typically utilize an air brake system having a pressure chamber, a push rod extending longitudinally outwards from the pressure chamber, a brake arm and a clevis pin for pivotally mounting the brake arm to the push rod. When the brake pedal is depressed, air pressure increases in the pressure chamber. As the air pressure increases, a diaphragm contained in the pressure chamber deforms and exerts an outward pressure on the push rod. The deformation of the diaphragm causes the push rod to move outwardly from the pressure chamber, actuating the braking mechanism. When air brake systems are properly adjusted, the push rod extends from a first position (in which the brakes are not engaged) to a second position (in which the brakes are fully engaged). Over time, as the brakes are repeatedly applied, the distance travelled by the push rod may increase requiring the brake system to be adjusted.
Some braking systems include slack adjusters. The slack adjuster is used to take up the excess clearance, i.e. the slack, caused by lining and drum wear. However, it is still important to regularly monitor the distance travelled by the push rod since this provides information regarding the degree of wear of the braking systems. Further, some jurisdictions require the operator of a truck to inspect the adjustment of the braking system before the vehicle's first trip each day.
Various devices have been developed to assist in monitoring the adjustment of brake systems.
U.S. Pat. No. 4,279,214 (Thorn) discloses a brake ware indicator comprising a sleeve that loosely fits over the push rod of the pneumatic actuator of the brakes. The sleeve has an outside diameter permitting it to be received on the aperture of the actuator housing which slidably receives the push rod.
U.S. Pat. No. 4,776,438 (Shandelmeier) discloses an air brake adjustment indicator comprising a bracket having an elongated body portion. A base plate is provided at one end for mounting on the vehicle. A pointer is adjustably mounted on the bracket at a predetermined distance from the clevis pin of the brake system in a direction opposite the air chamber when the clevis pin is in its normal, rest position.
U.S. Pat. No. 4,879,964 (Emerson, Jr.) discloses an adjustment marker comprising a chain of predetermined length which is attached to the push rod. A snap is attached to one end of the chain and a clamp is attached to the other end. An indicator flag is attached to the chain near the snap so that, when the push rod travels further than the predetermined length, the snap will release and drop the indicator flag downwardly so as to be visible to the operator of the vehicle.
U.S. Pat. No. 5,244,061 (Hoyt et al No. 1) discloses an air brake stroke length gauge comprising a stroke length indicator and a reference marker. The stroke length indicator is mounted on the push rod to move therewith. The reference marker is fixedly attached to the air brake system adjacent to the push rod at a distance which corresponds to the appropriate stroke length for the brake system when properly adjusted.
U.S. Pat. Nos. 5,320,198 (Hoyt et al No. 2) and 5,441,128 (Hoyt) disclose air brake stroke length adjustment gauges utilizing markers provided on the slack adjuster to indicate brake wear.
One disadvantage of some of these devices is that some portions of the indicator devices are subject to movement or may fall off with use. For example, when a vehicle is being driven, the frame of the vehicle and parts attached thereto (eg. the brake and the adjustment gauge) undergo substantial vibrations. This could result in the position of the reference marker and the stroke length indicator of Hoyt et al moving and accordingly providing a false reading.
A further disadvantage of some of these devices is that a warning is only provided when the push rod extends too far. For example, the device of Emerson, Jr. only provides a warning when the snap releases and the flag falls.
A further disadvantage of some of the devices is that it is difficult for the operator to properly read the adjustment gauge. Adjustment gauges are mounted near the push rod. If the operator reads the gauge from an angle, then a false reading may be obtained. In addition, some of these devices are not readily adjustable and substantial time may be required to properly mount and set the gauges.
A final disadvantage of some of these devices is that over the lifetime of the braking system, the clevis pin tends to seize in either the yokes of the pushrod or the yoke of the slack adjuster or the brake arm. If the truck is not in use, then the clevis pin may become seized in the slack adjuster/brake arm and/or the yoke. If the truck is in regular use, then continual applications of the brake will assist in preventing the clevis pin becoming seized in then slack adjuster/brake arm. However, the clevis pin may still become seized in the yoke.